The present invention generally relates to feedback type comb filters, and more particularly to a feedback type comb filter of a type having a feedback path for feeding back a part of an output of a delay circuit to an input of the delay circuit, which is designed so that a normal comb filter characteristic is obtained from a low-frequency range up to a high-frequency range.
Generally, as will be described later on in the specification in conjunction with drawings, a conventional feedback type comb filter comprises a delay circuit for delaying an input signal, a first adding circuit for adding the input signal and an output signal of the delay circuit and for producing an output signal as an output of the comb filter, a second adding circuit for adding the input signal and the output signal of the first adding circuit, and a coefficient multiplier provided in a feedback path from the output of the first adding circuit to the second adding circuit. When it is assumed that a delay time of the delay circuit in this conventional feedback type comb filter is equal to a time T.sub.P, the comb filter has a comb filter characteristic in which the pass bands are located at frequencies which are even number multiples of F.sub.P /2, where F.sub.P =1/T.sub.P, and the attenuation bands are located at frequencies which are odd number multiples of F.sub.P /2. Accordingly, when the value of F.sub.P is selected to a horizontal scanning frequency of a video signal, the comb filter may be used as a filter circuit for separating and filtering a luminance from the video signal, or may be used as a part of a noise reduction circuit for reducing a noise component within the luminance signal. When a subtracting circuit is instead of the first adding circuit described before, the comb filter may be used as a filter circuit for separating and filtering a carrier chrominance signal from the video signal.
In the conventional feedback type comb filter, a signal path is formed in which the input signal is applied to an input terminal, passed through the delay circuit, and is obtained through an output terminal. On the other hand, another signal path is formed in which the output signal of the delay circuit is passed through the coefficient multiplier, supplied to the delay circuit and passed through the delay circuit again, and is obtained through the output terminal. However, delay times of the two signal paths are different from each other. For this reason, peak frequencies in the pass band characteristics become shifted from regular frequencies. This shift of the peak frequencies from the regular frequencies, is more notable in the high-frequency range, and there is a problem in that a normal comb filter characteristic cannot be obtained.